Thin-film semiconductor devices such as thin-film transistors (TFTs) are conventionally used in active-matrix display devices such as liquid crystal display devices, as well as in solid state imaging devices such as digital cameras. In display devices, a TFT is used, for example, as a switching element for selecting a pixel, a driver transistor for driving a pixel, or as a driver external to the display region.
For example, an organic electroluminescent (EL) display including organic EL elements utilizing an organic light-emitting material differs from a voltage-driven liquid crystal display in that it is a current-driven display device. Therefore, development of a better performing TFT is rapidly progressing. Recent years have seen active development of a TFT using, as a channel layer, an oxide semiconductor, a representative example of which is InGaZnO (for example, see Patent Literature (PTL) 1).